Wood Beam Reinforcement through sistered members
Wood Beam Reinforcement through sistered members
(OP)
I have run into this case on numerous ocassions and want to make sure I'm looking at it correctly. I have an existing 4x12 beam that needs to be reinforced for a new loading. The new load produces a moment of 1763 ft-lb, and I will support this by sistering a 2x8 on either side of the beam. My question comes with the nailing required to make this beam act as a single unit and adequately resist the loads. I've gone through the horizontal shear calculation, but am not quite sure if this is the correct way to go about it. All examples I have seen assume that the beams stack on top of each other instead of side by side. I have attached my calculation so please take a look and let me know what you think. A few main questions.
1. Can I even use the horizontal shear equation when they're side by side?
2. Do I need to worry about the connection so much when they are side by side? I can see the merit when they stack vertically, but feel like they should act together with minimal nailing in this case. Maybe 2 rows of 10d at 6" o.c. staggered.
What is making me question myself is the 371 lb/in I am getting. I would need an excessive amount of nails to satisfy this shear when the extra load the beam will take appears to be rather small. Any thoughts?
1. Can I even use the horizontal shear equation when they're side by side?
2. Do I need to worry about the connection so much when they are side by side? I can see the merit when they stack vertically, but feel like they should act together with minimal nailing in this case. Maybe 2 rows of 10d at 6" o.c. staggered.
What is making me question myself is the 371 lb/in I am getting. I would need an excessive amount of nails to satisfy this shear when the extra load the beam will take appears to be rather small. Any thoughts?






RE: Wood Beam Reinforcement through sistered members
RE: Wood Beam Reinforcement through sistered members
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Wood Beam Reinforcement through sistered members
RE: Wood Beam Reinforcement through sistered members
RE: Wood Beam Reinforcement through sistered members
I found A* to be 1.5x7.25 = 10.875
y* = 4.92 - 7.25/2 = 1.295
Q = A*.y* = 14.08
On this basis, the horizontal shear is VQ/I = 3020 x 14.08/642.64 = 66#/"
That would be the total shear across the plane between each 2x8 and the 4x12, but it is not uniformly distributed. The fiber stress in each 2x8 varies from maximum tension at the bottom to a slight compression at the top of the 2x8, so the horizontal shear has a similar variation across the height of the 2x8.
You would need more nails near the bottom of the 2x8 than the top to handle this variation.
It would be better to laminate a 2x12 on each side of the 4x12 so that each lamination tends to have about the same deflection under its share of the load.
BA
RE: Wood Beam Reinforcement through sistered members
RE: Wood Beam Reinforcement through sistered members
BA
RE: Wood Beam Reinforcement through sistered members
BA
RE: Wood Beam Reinforcement through sistered members
I can see it both ways, and the answer depends on how the joists load the beam.
If the joists rest only on the top of the 4X12, then to get any load to the 2X8's, there must be horizontal nailing to estabish composite action to get any load to the 2X8's.
However, if the joists rest on top of the 4X12, and if there is solid blocking under the joists to the top of the 2X8's, then the load will be shared based on relative "I" values without the need for anything but nominal horizontal nailing.
The beam and 2X8's will deflect the same in either case here
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Wood Beam Reinforcement through sistered members
I(4x12)= 415.3
Portion carried by 2-2x8 = 95.3/(415.3 +95.3) = 0.186 or say 19% of the load (perhaps not negligible but not very much).
The OP will have to tell us what he means. I interpreted his attachment to mean that he was trying for the composite shape.
BA
RE: Wood Beam Reinforcement through sistered members
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Wood Beam Reinforcement through sistered members
Garth is right. Composite design in wood needs to be achieved by glue lamination, not by connectors.
RE: Wood Beam Reinforcement through sistered members
I do recall beefing up a 5" wide glulam beam some years back with a 5 x 3/8 plate pre-drilled with holes to receive glulam rivets and fastened to the underside of the beam. It did work with that type of fastener but it took a lot of elbow grease to install the fasteners.
If 19% is adequate, then the 2-2x8 fix may be sufficient but the OP has said that they cannot extend to the supports so special provisions would still be required to make the attachment.
BA
RE: Wood Beam Reinforcement through sistered members
RE: Wood Beam Reinforcement through sistered members
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Wood Beam Reinforcement through sistered members
RE: Wood Beam Reinforcement through sistered members
1. The original roof joists (2x4's) do not sit up on top of the 4x12 beam, but rather are attached on the side with hangers. This is the main reason that I am using 2x8's. I just don't have room to add in a full 12" deep section to match the beam. This being said, I can just tell the contractor to scoot the 2x8's up so that they create a bearing condition with the roof joists. This way we can be sure that the load will bear on both the 4x12 and 2x8's simultaneously, and then can just add in minimal nailing as long as deflections are considered. The bottoms of the 4x12 and 2x8's won't be flush like my sketch shows, but that really doesn't matter.
2. Just out of curiosity, how do you all think the connection would change if I weren't able to jam the 2x8's under the 2x4's so that they actually had bearing? Would this change the nailing requirements? The load path would go directly into the 4x12, and then have to be distributed out into the 2x8's so that is where I feel like a tighter nailing pattern would be required. Msquared seems to say that this case would require the horizontal nailing that I was initially trying to calculate and I agree.
Hope that helps. Let me know if I can provide any more info.
RE: Wood Beam Reinforcement through sistered members
RE: Wood Beam Reinforcement through sistered members
2x8 to 4x12, Qxx = -14.0 in3
Ixx total = 566.6 in4
Qxx/Ixx = -0.025 in-1
Integrate V over L = desired anchor spacing x Qxx/Ixx = Required Z for lag connector
This will give you composite action for bending. Verify that 4x12 can take the shear at the ends.
The 4x12 is already taking the existing load. Recommend relieving that load prior to sistering. Otherwise the 4x12 will carry the full existing load and then share with the 2x8 the added load (potential overstress on the 4x12).
In response to your questions:
1. Can I even use the horizontal shear equation when they're side by side? - Yes, if you want composite action with beams that do not have a common neutral axis. As noted above, you can always treat them separately, but your load distribution will be relative to their stiffnesses.
2. Do I need to worry about the connection so much when they are side by side? Yes, if you want composite action. In bending the 2x8s will take more tension which will be balaqnced by more compression in the 4x12.
RE: Wood Beam Reinforcement through sistered members
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Wood Beam Reinforcement through sistered members
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.